Don't be fooled - the camera isn't infallible, and can sometimes make mistakes that would make even the worst photographer look good. Well, technically, I suppose, the camera isn't making mistakes, but it is being fooled into making a poor decision.
Here's an example - I had a pigeon trying to steal my lunch while I was sitting in the park. He was a cheeky little blighter, and was sticking really close to me, no matter how much I tried to shoo him off, so I thought I'd get the camera out and try and get a worm's eye view shot. From this low vantage point, where I couldn't get my head down low enough to check the viewfinder, I had to point and hope that the auto-everythings worked for me, but no, the camera decided that despite the pigeon being right in the middle of the picture, the people in the background were obviously what I wanted to focus on ;-) Thinking about it, I have never had a clear picture of a pigeon - I wonder if my camera is prejudiced?